Transmission line surveillance system measuring changes in phase of propagated signals

ABSTRACT

A surveillance system in which a cyclically varying signal is launched into a data carrying optical fibre (3). The signal is reflected from the far end of the fibre and the returning signal compared with the launched signal. A change in the phase difference between the launched and returning signal is indicative of a change in length of the fibre. A microprocessor (8) is used to discount environmentally induced changes caused by temperature or traffic vibration. Alternatively the cyclically varying signal may also be transmitted to the far end of the fibre in digital form and after conversion used as the reference for comparison at the far end.

This invention relates to monitoring of optical fibres.

There are in existence surveillance systems for optical fibre networksoperating on optical time domain reflectometry (OTDR) principles. Inthese systems a high power pulse is launched into the optical fibre,this pulse travels along the fibre and is scattered by crystalinhomogeneities in the fibre, some of the scattering being backwardsalong the fibre to the end from which the pulse was launched. At thislaunch end the returning backscattered light is monitored for a periodfollowing launch of the pulse. The returning backscattered lighteffectively constitutes a series of `echoes` and it is possible todetermine the power level of the pulse at a point on the fibre becausethe power level bears a relationship to the strength of the returningbackscattered signal, and the time interval between launch of the pulseand the respective return `echo` provides, from a presumption of thepropagation time, an indication of the distance along the fibre fromwhich the light is being scattered. Over a period of time a lowering inthe echo strength related to a particular location on the fibre isindicative of power loss which may be indicative of unauthorisedcoupling out of light, but may also be caused by other factors. Adisadvantage of using OTDR monitoring for surveillance is that the highpower pulse sources required are costly and bulky. Also although powerloss is detectable, the system does not provide an indication ofunauthorised handling or proximity to transmission lines, for examplefor the purpose of sabotage, until actual damage occurs. It is alsopossible for power to be tapped and simultaneously injected withoutdetection.

The present invention is concerned with providing a sensitivesurveillance system that may also provide an early indication ofunauthorised access to an optical fibre network.

Accordingly the invention provides a surveillance method comprisinggenerating a cyclically varying signal, modulating an optical beam withsaid signal and launching the beam into an optical fibre, comparing atleast part of the signal after transmission along the fibre with areference signal related to the cyclically varying signal and monitoringphase changes in the cyclically varying signal occurring in transmissionalong the fibre.

Within the context of this specification the expression `optical` is notrestricted in its reference to the visible portion of theelectromagnetic spectrum, and especially it includes the infra red andwavelengths transmittable by optical fibres.

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a schematic illustration of a first embodiment of theinvention in which a surveillance signal is launched into a fibre andmonitored from the launch end, and

FIG. 2 is a schematic illustration of a second embodiment in which thesurveillance signal is monitored at the end remote from the launch ofthe surveillance signal.

In local or short haul communication networks there is a demand forconfidential transmission and it is this type of network that is theprinciple concern of the invention although it is envisaged that thesystem may be developed for longer haul applications. In a preferredembodiment a surveillance signal is generated and transmitted along thesame path as the data signals. Any available transmission window may beused for the surveillance signal, for example the data transmission mayuse 1300 nm and the surveillance signal 1310 nm or data tansmissions maybe spread around 1300 nm and the surveillance signal transmitted at 850nm. The surveillance signal is constantly transmitted even when there isno data transmission.

The surveillance signal consists of a cyclically varying signal, forexample a laser beam having a square or sinusoidally modulatedamplitude. In fact the modulation may take any form in which phasedifference or delay can be detected, and for a given maximum amplitudethe shorter the modulation period the more sensitive it becomes to phasedifference. This amplitude modulated signal is launched into thetransmission fibre and propagates along the fibre until it is reflectedback towards the launching end by the fibre termination at the far endof the fibre. The proportion of the surveillance signal that isreflected may be increased by providing a metallised coating on thefibre termination or by provision of a grating or other wavelengthsensitive device that will selectively reflect the surveillance signal.At the launching end the returning surveillance signal is compared withthe launched signal for phase difference and this phase differenceprovides a reference value. If the transmission path is elongated thenthe phase difference between the launched and returning signals changesand this change is significant even for minute elongation: picking up asingle fibre or disturbing a fibre by blowing on it is sufficient tochange the phase difference between the launched and returningsurveillance signals at surveillance signal frequencies of the order of200 MHz. Thus by monitoring for phase difference changes from thereference value an indication of change in length of the fibretransmission line can be detected and this may indicate disturbance ofthe line. In general it is envisaged that frequencies greater than 50MHz will be used, preferably frequencies in excess of 100 MHz and lessthan 300 MHz.

Various factors other than disturbance may also cause changes in lengthof the fibre, for example thermal expansion and contraction andvibration caused by the environment such as the passage of traffic. Inorder to eliminate consideration of changes due to these factors thephase difference values are monitored by a microprocessor which revisesthe reference value taking into account changes in temperature, trafficflow conditions and other variations related to external conditions orfollowing predictable patterns. For example, if ambient temperatureincreases this can be monitored directly or, as will be necessary if thechange in temperature occurs at a part of the transmission line remotefrom the microprocessor or any related sensor, the microprocessor willtake into account comparatively slow and steady changes in the phase andrecognise this as a temperature variation pattern and revise thereference value. Likewise vibration patterns caused by traffic will berecognisable both from time of day expectation levels and from themagnitude of the shifts. In contrast to this in order to tap into afibre transmission line it is necessary to expose the fibre in order tocouple it to a tapping fibre and this operation necessarily disturbs thefibre to a much greater extent than mere vibration because the ductcarrying the fibre package and the fibre package itself have to beopened up. This type of disturbance would not follow a recognisable orexpected regular pattern and the microprocessor would shut down thecommunication channel or provide other indications that unusualtampering may have occurred. In some locations the vibration patterncaused by approach to the fibre without actual hand ling may besufficiently different from normal ambient conditions to raise an alertcondition. The sensitivity of the system may also be selected by choiceof surveillance signal frequency, the higher the frequency the moresensitive the system, so that a particular level of disturbance isnecessary for detection and this can eliminate consideration of smallvibrations for example.

In the system described so far the surveillance signal has beentransmitted along the same fibre as the data. In practice it is usualfor a transmission line package to include several separate opticalfibres and as these packages are sealed the whole package is disturbedwhen access to a component fibre is made, so that a surveillance signalcarried by one fibre in a package provides surveillance for the wholepackage. The technique may also be utilised as just a disturbancemonitor without data transmission. In some instances greater sensitivitymay be achieved by reducing the extent of packaging around a portion offibre so as to render it more sensitive to its environment, andespecially to vibration caused by approach.

It may be desirable to monitor for phase change at the end of the lineremote from launch by comparing the incoming surveillance signal with areference signal that is locally generated or transmitted to that endalong the same or a different route. With this modification surveillanceof more than one span of fibre may be achieved at a central monitoringlocation.

In a single ended (launch end monitored) system the return signal may betransmitted along a different return fibre to the outbound signal.

Referring now to the drawings, FIG. 1 shows a first preferred embodimentof the invention. In this embodiment a surveillance signal is generatedby a signal generator 1 and this signal modulates the output of an LEDor laser source 2 which is launched into an optical fibre transmissionline 3, via a coupler 4. The line 3 may be a fibre that is also carryingdata signals (as shown) from a data transmission source 6 or a separatefibre for surveillance only. The signal launched into the fibre 3propagates to the remote end of the transmission line where it isreflected or returned back into the fibre by reflector 4 which may be amirror, cleaved end, loop and coupler or such like.

If the line 3 is also carrying data, then at the far end there is asecond coupler 5 which splits the light with one output going to datadecoding and the other to the reflector 4. The reflected signal returnsvia coupler 5 (when required) along the line 3 to coupler 4 from whichit is then input to a phase lock amplifier 7. The data signal (whenpresent) is at a different wavelength λ₁ from the surveillance signalλ₂, and λ₂ is rejected at the phase lock amplifier. The amplifier 7 alsoreceives an input from the signal generator 1 and compares the phases ofthe original signal with the returning signal and inputs the comparisonto a microprocessor 8. In the microprocessor the phase change or delayis analysed and if it is found to be indicative of tampering themicroprocessor signals this to a data controller 9 which in turngenerates an audible alarm signal from alarm 10 and transmits a `disabledata` signal to the data transmitter 6, along line 3 and to the far endwhere transmission from data transmitter 11 is also disabled. A feedbackloop from the microprocessor inputs phase adjustment information due forexample to thermal expansion or other memorised information.

FIG. 2 shows a system, again for simultaneous transmission of data andsurveillance signal, in which the surveillance signal is monitored atthe far end from launch. In this system the signal generator's signal isalso input to a frequency to digital convertor 12 and transmitted at thedata frequency. At the far end the data frequency signal splits and thesignal generator frequency is recovered by a digital to frequencyconvertor 13 and this is input to the phaselocked amplifier 7 which alsoreceives the surveillance signal from coupler 4. The comparison,microprocessor and data disable operate similarly to that explained withreference to FIG. 1.

The data disable signal may take several forms. It may comprise aspecific signal or cessation of an all-clear signal. Likewise, onreceipt of the data disable signal, the data transmsitter may eitherchange or interrupt the data transmission.

I claim:
 1. A surveillance method comprising the following steps:(a)generating a cyclically varying signal, (b) modulating an optical beamwith said signal and launching the beam into an optical fibre, (c)comparing at least part of the signal after transmission along saidfibre with a reference signal related to the cyclically varying signalso as to determine a phase difference, (d) later repeating said steps(a)-14 (c) to determine a further phase difference, and (e) determiningthe occurrence of changes in the phase of the cyclically varying signaloccurring in transmission along the fibre in response to said determinedphase differences.
 2. A surveillance method according to claim 1 inwhich at least part of the cyclically varying signal is reflected backto the launching end of the fibre and is compared with the referencesignal at the launching end.
 3. A surveillance method according to claim1 or claim 2 in which the reference signal comprises the cyclicallyvarying signal prior to transmission along the fibre.
 4. A surveillancemethod according to claim 1 in which the cyclically varying signal isdigitally encoded, transmitted to the end of the fibre remote from thelaunching end and decoded to provide the reference signal, and thecomparison and monitoring takes place at the end of the fibre remotefrom the launch end.
 5. A surveillance method according to any precedingclaim 1 or 2 in which the cyclically varying signal comprises asubstantially continuously varying amplitude modulated signal.
 6. Asurveillance method according to any preceding claim 1 or 2 in which inthe event of phase changes exceeding a predetermined level informationdata transmitted over a transmission line under surveillance isdisabled.
 7. A surveillance method according to any preceding claim 1 or2 in which in the event of phase changes exceeding a predetermined levela warning signal is generated.
 8. A surveillance method according to anypreceding claim 1 or 2 in which the optical beam is provided by anelectroluminescent semiconductor device.
 9. A surveillance methodaccording to any preceding claim 1 or 2 in which the cyclically varyingsignal has a frequency of at least 100 MHz.
 10. A surveillance methodaccording to any preceding claim 1 or 2 in which phase changesconsequent on environmental patterns are automatically discounted.
 11. Asurveillance apparatus of the type which surveys an optical fibertransmission path, said apparatus comprising:generating means forgenerating a cyclically varying signal, means coupled to said generatingmeans and to said optical fiber transmission path for modulating anoptical beam with said cyclically varying signal and for coupling themodulated beam into said optical fiber transmission path, comparingmeans coupled to said optical fiber transmission path for receiving atleast portion of the signal after propagation of said signal along atleast a portion of said optical fiber transmission path, and forcomparing the phase of said received signal portion with the phase of areference signal related to the cyclically varying signal so as todetermine phase differences, and means coupled to said comparing meansfor determining, in response to the results of plural such phasecomparisons by said comparing means, the occurrence of phase differencechanges between the phase of said cyclically varying signal and thephase of said reference signal.
 12. Apparatus for detecting tamperingwith an optical fiber transmission path, said apparatus comprising:atransmitter coupled to said optical fiber transmission path, saidtransmitter coupling light modulated by a cyclically varying signal intosaid optical fiber transmission path, said modulated light propagatingalong said path; a receiver coupled to said optical fiber transmissionpath, said receiver receiving a portion of said propagated modulatedlight; and processing means, coupled to said receiver, for continuallymonitoring the phase of said propagated modulated light, for detectingchanges in said monitored phase over time, and for generating an outputsignal indicating said optical fiber transmission path has been tamperedwith in response to unexpected detected phase changes over time. 13.Apparatus as in claim 12 wherein said processing means includes meansfor generating said output signal only if a detected phase changeexceeds a predetermined change.
 14. Apparatus as in claim 12 whereinsaid processing means includes means for adapting to gradual changes inthe condition of said optical fiber transmission path.
 15. Apparatus asin claim 12 wherein said modulated signal is at a first frequency andsaid transmitter further includes means for injecting a data signal at asecond frequency different from said first frequency into said opticalfiber transmission path.
 16. Apparatus as in claim 15 wherein saidoptical fiber transmission path has a first end and a second end, saidtransmitter and receiver are both coupled to said optical fibertransmission path first end, and said apparatus further includesfrequency selective reflecting means, coupled to said optical fibertransmission path second end, for reflecting said first frequency signaland for passing said second frequency signal.
 17. Apparatus as in claim12 further including means, operatively coupled to said transmitter andto said processing means, for disabling said transmitter in response tosaid output signal.
 18. Apparatus as in claim 12 wherein saidtransmitter constantly applies said signal to said optical fibertransmission path.
 19. A surveillance method comprising generating acyclically varying signal having a modulation frequency of at least 50MHz, modulating an optical beam with said signal and launching the beaminto an optical fibre, comparing at least part of the signal aftertransmission along the fibre with a reference signal related to thecyclically varying signal and monitoring phase changes in the cyclicallyvarying signal occurring in transmission along the fibre.
 20. Asurveillance method for an optical fibre transmission system, the methodhaving a sensitivity great enough to detect remotely the non-damaginghandling of an optical fibre which is under surveillance, the methodcomprising generating a cyclically varying signal, modulating an opticalbeam with said signal and launching the beam into said optical fibre,comparing at least part of the signal after transmission along saidfibre with a reference signal related to the cyclically varying signaland monitoring phase changes in the cyclically varying signal occurringin transmission along said fibre.
 21. A surveillance method for anoptical fibre transmission system, the method comprising generating acyclically varying signal having a modulation frequency of at least 50MHz, modulating an optical beam with said signal and launching the beaminto an optical fibre, comparing at least part of the signal aftertransmission along said fibre with a reference signal related to thecyclically varying signal and monitoring phase changes in the cyclicallyvarying signal occurring in transmission along said fibre, thesurveillance method being capable of detecting remotely the non-damaginghandling of said optical fibre merely from monitoring said phasechanges.
 22. A surveillance method according to any one of the precedingclaims 19-21 in which at least part of the cyclically varying signal isreflected back to the launching end of the fibre and is compared withthe reference signal at the launching end.
 23. A surveillance methodaccording to any one of the preceding claims 19-21 wherein at least partof the cyclically varying signal is reflected back to the launching endof the fibre from a remote end of the fibre and that part is comparedwith the reference signal at the launching end.
 24. A surveillancemethod according to any one of the preceding claims 19-21 in which thereference signal comprises the cyclically varying signal prior totransmission along the fibre.
 25. A surveillance method according to anyone of the claims 19-21 in which the cyclically varying signal isdigitally encoded, transmitted to the end of the fibre remote from thelaunching end and decoded to provide the reference signal, and thecomparison and monitoring takes place at the end of the fibre remotefrom the launch end.
 26. A surveillance method according to anypreceding claim 19-22 in which the cyclically varying signal comprises asubstantially continuously varying amplitude modulated signal.
 27. Asurveillance method according to any preceding claim 19-21 in which inthe event of phase changes exceeding a predetermined level, informationdata transmitted over a transmission line under surveillance isdisabled.
 28. A surveillance method according to any preceding claim19-21 in which in the event of phase changes exceeding a predeterminedlevel, a warning signal is generated.
 29. A surveillance methodaccording to any preceding claim 19-21 in which in the optical beam isprovided by an electroluminescent semiconductor device.
 30. Asurveillance method according to any preceding claim 19-21 in which thecyclically varying signal has a frequency of at least 100 MHz.
 31. Asurveillance method comprising generating a cyclically varying signal,modulating an optical beam with said signal and launching the beam intoan optical fibre, comparing a non-backscattered signal which correspondswith at least part of the signal after transmission along the fibre witha reference signal related to the cyclically varying signal andmonitoring phase changes in the cyclically varying signal occurring intransmission along the fibre.
 32. An intruder alarm system for anoptical fibre communication system comprising:an optical waveguide;firstmeans to generate a cyclically varying signal; second means to modulatean optical beam with said signal; third means disposed adjacent andoptically coupled to one end of said waveguide to inject an optical beamso modulated into said one end of said waveguide for transmission to theother end thereof; fourth means to compare at least part of the signalafter transmission along the fibre with a reference signal related tothe cyclically varying signal; and fifth means for monitoring phasechanges in the cyclically varying signal which occur in transmissionalong the fibre and for providing an alarm function in the event ofphase changes exceeding a predetermined level.